A. Field of the Invention
The present invention relates to wood flake molding.
B. Background of the Art
Wood flake molding, also referred to as wood strand molding, is a technique invented by wood scientists at Michigan Technological University during the latter part of the 1970s for molding three-dimensionally configured objects out of binder coated wood flakes having an average length of about 1¼ to about 6 inches, preferably about 2 to about 3 inches; an average thickness of about 0.005 to about 0.075 inches, preferably about 0.015 to about 0.030 inches; and an average width of 3 inches or less, most typically 0.25 to 1.0 inches, and never greater than the average length of the flakes. These flakes are sometimes referred to in the art as “wood strands.” This technology is not to be confused with oriented strand board technology (see e.g., U.S. Pat. No. 3,164,511 to Elmendorf) wherein binder coated flakes or strands of wood are pressed into planar objects. In wood flake or wood strand molding, the flakes are molded into three-dimensional, i.e., non-planar, configurations.
In wood flake molding, flakes of wood having the dimensions outlined above are coated with MDI or similar binder and deposited onto a metal tray having one open side, in a loosely felted mat, to a thickness eight or nine times the desired thickness of the final part. The loosely felted mat is then covered with another metal tray, and the covered metal tray is used to carry the mat to a mold. (The terms “mold” and “die”, as well as “mold die”, are sometimes used interchangeably herein, reflecting the fact that “dies” are usually associated with stamping, and “molds” are associated with plastic molding, and molding of wood strands does not tit into either category.) The top metal tray is removed, and the bottom metal tray is then slid out from underneath the mat, to leave the loosely felted mat in position on the bottom half of the mold. The top half of the mold is then used to press the mat into the bottom half of the mold at a pressure of approximately 600 psi, and at an elevated temperature, to “set” (polymerize) the MDI binder, and to compress and adhere the compressed wood flakes into a final three-dimensional molded part. The excess perimeter of the loosely felted mat, that is, the portion extending beyond the mold cavity perimeter, is pinched off where the part defining the perimeter of the upper mold engages the part defining the perimeter of the lower mold cavity. This is sometimes referred to as the pinch trim edge.
U.S. Pat. Nos. 4,440,708 and 4,469,216 disclose this technology. The drawings in U.S. Pat. No. 4,469,216 best illustrate the manner in which the wood flakes are deposited to form a loosely felted mat, though the metal trays are not shown. By loosely felted, it is meant that the wood flakes are simply lying one on top of the other in overlapping and interleaving fashion, without being bound together in any way. The binder coating is quite dry to the touch, such that there is no stickiness or adherence which hold them together in the loosely felted mat. The drawings of U.S. Pat. No. 4,440,708 best illustrate the manner in which a loosely felted mat is compressed by the mold halves into a three-dimensionally configured article (see FIGS. 2-7, for example).
This is a different molding process as compared to a molding process one typically thinks of, in which some type of molten, semi-molten or other liquid material flows into and around mold parts. Wood flakes are not molten, are not contained in any type of molten or liquid carrier, and do not “flow” in any ordinary sense of the word. Hence, those of ordinary skill in the art do not equate wood flake or wood strand molding with conventional molding techniques.
Some parts are designed with holes in them, for example, for receiving so called “T” nuts. T nuts comprise an annular flange with one or more barbs projecting therefrom for anchoring the T nut in the surface of the part, and with an internally threaded sleeve extending perpendicularly from the annular flange. Thus, the hole in the part must extend perpendicularly back from the surface of the part. Such holes may also be required for other purposes.
While such holes can be drilled into a molded wood strand part after it is molded, it would be preferable to mold the holes into the part as part of the molding process. To do so, the mold cavity is provided with a punch projecting from one surface thereof, and a hole defining punch receiving insert in the opposite mold cavity surface, such that as the upper and lower mold halves, or platens, are closed, the punch pushes through the loosely felted mat, pushing wood flakes aside or bending them in the direction of the punch, such that as the binder cures, a hole is formed around the punch.
Heretofore, the holes have been with a punch having a tapered outer surface that pushes past and through the wood strands in the felted, mat. The punch is tapered to facilitate its removal from the molded part. However, due to the nature of wood strand molding, the final molded part can vary ill thickness. The mats can be compressed to varying thicknesses by the mold die, due to unavoidable inconsistencies from mat to mat, springs back of the mat, overcompression, or file like. Therefore, the dimensions of an opening on one side of a molded hole formed by a tapered punch can vary with variations in the final thickness of the part, depending on the distance the tapered punch extends through the molded part. Therefore, while the openings on one side of a hole may have the same radius for all molded parts using that punch, the openings on the other side of the hole will have varying radii, depending on the thickness of the molded part.